FIG. 1 is a view showing a network structure of a universal mobile telecommunication system (UMTS).
The UMTS largely includes a user equipment (UE), a UMTS terrestrial radio access network (hereinafter, abbreviated to UTRAN), and a core network (hereinafter, abbreviated to CN). The UTRAN includes at least one radio network sub-system (hereinafter, abbreviated to RNS). Each RNS includes a radio network controller (hereinafter, abbreviated to RNC) and at least one base station (hereinafter, referred to as a Node B) managed by the RNC. One Node B includes at least one cell.
FIG. 2 is a view showing the structure of a radio interface protocol between the UTRAN and the UE on the basis of 3GPP radio access network standard.
As shown in FIG. 2, the radio access interface protocol includes horizontal layers including a physical layer, a data link layer and a network layer, and vertical planes including a user plane for transmitting data information and a control plane for transmitting control signals. In FIG. 2, the protocol layers can be divided into L1 (a first layer), L2 (a second layer), and L3 (a third layer) based on three lower layers of an open system interconnection (OSI) standard model well known in the art of communication systems.
Hereinafter, the layers shown in FIG. 2 will be described. The physical layer which is the first layer provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to a medium access control layer, which is an upper layer, through a transport channel, and data is transferred between the medium access control layer and the physical layer through the transport channel. In addition, data is transferred between different physical layers, that is, physical layers of a transmission side and a reception side, through a physical channel.
The medium access control (hereinafter, abbreviated to MAC) layer, which is the second layer, provides a service to a radio link control layer, which is an upper layer, through a logic channel. The radio link control (hereinafter, abbreviated to RLC) layer, which is the second layer, provides support for reliable data transmissions, and may perform a function of segmentation and concatenation of an RLC service data unit (SDU) coming from a higher layer.
A radio resource control (hereinafter, abbreviated to RRC) layer located at a lowest portion of the third layer is only defined in the control plane, and controls the logic channels, the transport channels and the physical channels in relation to the configuration, the reconfiguration, and the release of a radio bearer (Hereinafter, abbreviated to RB). At this time, the RB signifies a service provided by the second layer for data transmission between the UE and the UTRAN. In general, the set up of the RB refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific service, and setting the respective detailed parameters and operation methods.
A RACH of a wideband code division multiple access (WCDMA) will be described in more detail as follows. The RACH is used to transfer short length data on an uplink. In more detail, the RACH is used when the UE acquires initial uplink synchronization. The RACH is used when the UE is first turned on or is switched from a long-time idle mode to an active mode such that the uplink synchronization is set again, and may be used without establishing time synchronization or frequency synchronization. The RACH basically supports multiple users. Each UE transmits a specific preamble sequence when accessing the RACH, the Node B recognizes the preamble sequence and transmits a signal to a downlink, and the UE updates its own time synchronization information using the information. At this time, if frequency synchronization information is transmitted together, the frequency synchronization information may be used in the information of the UE.
The RACH, which is the transport channel, is mapped to the physical random access channel (PRACH).
FIG. 3 is a view showing a conventional PRACH transmission.
As shown in FIG. 3, the PRACH is divided into a preamble part and a message part. The preamble part performs a power ramping function for properly controlling transmission power used for message transmission and a function for avoiding collision among several UEs. The message part performs a function for transmitting an MAC protocol data unit (hereinafter, abbreviated to PDU) transferred from the MAC to the physical channel.
When the MAC of the UE instructs a PRACH transmission to the physical layer of the UE, the physical layer of the UE first selects one access slot and one signature, and transmits the preamble on the PRACH to an uplink. The preamble is transmitted within an access slot duration having a length of 1.33 ms, and one signature is selected from 16 signatures within a first certain length of the access slot and is transmitted. When the UE transmits the preamble, the Node B transmits a response signal through an acquisition indicator channel (AICH) which is a downlink physical channel. The AICH, in response to the preamble, transmits the signature which was selected by the preamble within the first certain length of the access slot corresponding to the access slot for transmitting the preamble. At this time, the Node B transmits an acknowledge (ACK) response or a non-acknowledge (NACK) response to the UE through the signature transmitted by the AICH.
When the UE receives the ACK response, the UE transmits a message part having a length of 10 ms or 20 ms using an orthogonal variable spreading factor (OVSF) corresponding to the transmitted signature.
When the UE receives the NACK response, the MAC of the UE instructs a PRACH retransmission to the physical layer of the UE after a certain time period. In contrast, if the UE does not receive the AICH corresponding to the transmitted preamble, the UE transmits a new preamble with power higher than that of the previous preamble, after a predetermined access slot.
FIG. 4 is a view showing an exemplary structure of an AICH which is a conventional downlink physical channel.
The AICH, which is the downlink physical channel, transmits 16 symbol signatures (Si, i=0 . . . 15) for the access slot having a length of 5120 chips. Here, the UE may select any arbitrary signature (Si) from S0 signature to S15 signature, and then transmits the selected signature during the first 4096 chips length. The remaining 1024 chips length is set as a transmission power off period during which no symbol is transmitted. Also, as similar to FIG. 4, the preamble part of the PRACH, which is the uplink physical channel, transmits 16 symbol signatures (Si, i=0 . . . 15) during the first 4096 chips length.
However, in the conventional RACH transmission, since the downlink message, which is transmitted in response to the uplink RACH message transmission, is transmitted with high power so as to be received even in a cell boundary, a radio resource was inefficiently used.